I have two 300 gallon GB's with a 850 gallon fish tank . My Pipe Siphons are made from 1 ½ " PVC Pipe, Tee’s & 90’s . After media GB's hold probably 150 of water, it takes 8 mins. to fill and 4 mins. to drain. If your new GB's are 55 gal drums cut in 1/2, you're only draining about 12 gals of water. You can build the Pipe Siphon any size you want...I was thinking that 3/4" PVC might be even to large for your set up.

A lot of newer builds are installing a swirl filter between your FT and your GB's. It cuts down on the s___ build up in your GB's. I'm thinking about building a pressurized one for my set up, I will run PVC over to my old style garden...install it over there. I can then use the fertilized water for those plants and to lower my nitrates in my AP if they get to high. I'll do my water exchanges...adding rain water from my rain barrels to my FT...releasing water from my swirl filter into my garden.